US 7038875 B2 Abstract The present invention is a novel method and apparatus for determining head media modulation in a magnetic data storage and retrieval system. The magnetic data storage and retrieval system includes a magnetic disc with a stored signal and a transducing head for reading a readback signal based on the stored data. The system includes a data acquisition and processing circuit. The processing circuit generates a dynamic harmonic ratio for the readback signal. The processing circuit also generates a head media modulation signal as a function of time from the dynamic harmonic ratio.
Claims(20) 1. A magnetic data storage and retrieval system comprising:
a rotatable magnetic disc having a readback signal stored over a section of the magnetic disc;
a transducing head for reading the readback signal from the magnetic disc, the transducing head being positioned to fly at a fly height with respect to the magnetic disc when the magnetic disc is rotated;
means for processing the readback signal to calculate a dynamic harmonic ratio as a function of time; and
means for calculating head media modulation as a function of time from the dynamic harmonic ratio.
2. The magnetic data storage and retrieval system of
3. The magnetic data storage and retrieval system of
4. The magnetic data storage and retrieval system of
5. The magnetic data storage and retrieval system of
6. A magnetic data storage and retrieval system comprising:
a rotatable magnetic disc having data stored therein;
a transducing head for reading a readback signal representative of the data from the magnetic disc as the transducing head is flying above the magnetic disc as the magnetic disc is rotated;
a data acquisition circuit for digitizing the readback signal; and
a processing circuit for (a) calculating a frequency spectrum of the readback signal for each of multiple selected sampling intervals of the readback signal, (b) calculating a harmonic ratio for each of the selected sampling intervals based on the calculated frequency spectrum for each of the selected sampling intervals, (c) generating a dynamic harmonic ratio for the readback signal from the harmonic ratio calculations, and (d) generating a head media modulation signal as a function of time from the dynamic harmonic ratio.
7. The magnetic data storage and retrieval system of
8. The magnetic data storage and retrieval system of
9. The magnetic data storage and retrieval system of
10. The magnetic data storage and retrieval system of
11. The magnetic data storage and retrieval system of
12. The magnetic data storage and retrieval system of
13. A method for determining a dynamic harmonic ratio for an entire readback signal in a data storage and retrieval system comprising:
rotating a magnetic disc having a readback signal stored therein;
reading the readback signal from the magnetic disc with a transducing head being positioned above the magnetic disc when the magnetic disc is rotated;
storing the readback signal;
calculating a frequency spectrum of the readback signal for each of multiple selected sampling intervals of the readback signal;
calculating a harmonic ratio for each of the selected sampling intervals based on the calculated frequency spectrum for each of the selected sampling intervals; and
generating a dynamic harmonic ratio for the readback signal from the harmonic ratio calculations; and
generating a head media modulation signal as a function of time from the dynamic harmonic ratio.
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Description None. The present invention relates to magnetic data storage and retrieval systems. In particular, the present invention relates to measuring head media spacing modulation. As areal density of magnetic data storage systems continues to increase, it is critical to maintain a controlled spacing between the magnetic recording head and the recording media. The head media spacing modulation is a measure of the displacement of the head relative to the media and is an important aspect of the fly condition of a slider. Head media spacing modulation is often caused by media waviness, disc clamping distortion, write current induced pole tip protrusion, micro-actuator induced modulation, as well as other factors. Often, it is desired to measure head media spacing to detect, for example, where there is a wave or defect in the media. One such indirect measurement is with the use of a laser doppler vibrometer (LDV). A LDV is mounted on the slider of a disc drive and it measures the slider motion on the backside of a slider. The process essentially first measures head media spacing modulation and disc motion in a vertical direction while the slider is flying at a normal height, and then second, measures the disc vertical motion without flying the slider. The difference between the two measurements is the head media spacing modulation. This indirect measurement cannot measure the actual spacing modulation at the pole tip and cannot measure the spacing change induced by write current. Another technique used to measure head media spacing modulation is with use of a harmonic ratio. Such a technique to measure head media clearance is described in U.S. Pat. No. 4,777,544, which is incorporated by reference herein. With use of that disclosed method, the harmonic ratio of the readback signal is measured at normal fly height. The slider and head are then lowered to near contact by lowering disc speed, and the harmonic ratio is measured again at near contact. The head media clearance is then calculated at discrete locations based on the two harmonic ratio measurements. This static measurement of head media spacing can give a good measurement of the distance of the head to the media at any particular specified location. An improved technique for measuring head media modulation is desired. The present invention is a novel method and apparatus for determining head media modulation in a magnetic data storage and retrieval system. The magnetic data storage and retrieval system includes a magnetic disc with a stored signal and a transducing head for reading a readback signal based on the stored data. The system includes a data acquisition and processing circuit. The processing circuit forms sampling intervals and calculates a harmonic ratio for the sampling intervals. The processing circuit generates a dynamic harmonic ratio for the readback signal using the harmonic ratio calculations. The processing circuit also generates a head media modulation signal as a function of time from the dynamic harmonic ratio. The present invention is particularly contemplated for use in a magnetic data storage and retrievable system such as disc drive A signal is stored on magnetic disc The readback signal contains a fundamental frequency and higher harmonics. The ratio of the harmonic components is sensitive to head media spacing but less sensitive to environmental effects. According to Wallace Spacing Loss Model, the ratio of the third and the first harmonics are related to head media spacing by the following equation:
DAPS Once the readback waveform is digitized, the data is processed according to a selected sampling interval. The frequency spectrum of the readback signal over the selected sampling interval is calculated as depicted in block DAPS DAPS Next, data acquisition and processing system In the present example, the harmonic ratio for the selected sampling interval was calculated by dividing the amplitude of the third harmonic frequency of the readback signal by the amplitude of the fundamental frequency of the readback signal. Any number of different harmonics and the fundamental frequency can also be used, to calculate the harmonic ratio and compile the dynamic harmonic ratio. In forming the dynamic harmonic ratio, the sampling interval selected must be a small enough increment of time such that there is sufficient resolution to the dynamic harmonic ratio for the selected readback waveform in order to determine the frequency spectrum of the dynamic harmonic ratio. For example, in The calculated dynamic harmonic ratio can be converted to head media modulation with Equation 2 described above. For example, Given the relatively small-sized amplitude of the dynamic harmonic ratio, filtering out noise is especially important. Often, signal amplitude due to noise can be larger in amplitude than the actual modulation signal. Identification of the modulation frequency allows the modulation signal to be isolated from any modulation due to noise or other affects. In this way, the actual head modulation as a function of time can be analyzed over a particular time period of interest. In calculating the instantaneous peak amplitude in the frequency spectrum of a sampling interval of the readback signal, limitations on signal length and frequency resolution affect the accuracy of the captured signals. Consequently, the true harmonic amplitude does not necessarily fall on the sampled points in the spectrum. For example, in The measurement of head media modulation in accordance with the present invention has many advantages. By calculating dynamic harmonic ratio and head media modulation while introducing lateral head micro-actuation, the affect of the lateral head micro-actuation can be measured. Micro-actuation induced modulation typically has lower frequency and larger amplitude and this makes the measurement more accurate than high frequency low amplitude air bearing modulation. Another useful application of dynamic harmonic ratio is in measurement of thermal pole tip protrusion. As shown in Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Patent Citations
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